It is common knowledge that fluids such as oil and natural gas are produced from permeable, porous, subsurface formations. The porosity of a formation is a measure of its ability to store oil and/or gas, whereas its permeability is an indication of the conductivity of the fluids contained in such formation.
Generally, the procedure involved in making an oil or gas well is to drill a hole from the surface down to the desired production zone and then provide the hole with casing. The casing-in of a hole consists in running a casing string down to a point close to its bottom and then pumping cement under pressure down the casing string and up the annular space between the casing and the wall of the bore hole.
The cement is subsequently allowed to set for a considerable length of time, in order to optimize its capability of forming a bond impermeable to water and oil between the casing and the bore wall. Once the cement has hardened, a special tool is run downhole and positioned at the reservoir depth, where it is caused to fire bullets which will perforate the casing, the cement sheath and even the rock, in the reservoir depth, in order to permit the flow of oil and gas through the production zone and the casing perforations and into the casing string.
During the production period of a well, it is often necessary to boost its hydrocarbon output or stimulate its production. Many reservoirs which require stimulation treatment do not show homogeneous permeability, that is, they either consist of layers of variable permeabilities or contain fractures which cause a treatment fluid to advance in the formation in a non-uniform manner. For instance, a highly permeable region in the formation will consume most of the stimulation treatment, leaving a less permeable region deprived of such treatment.
To overcome this problem, it is necessary to arrange a means of diverting the treatment fluid from the most permeable into less permeable zones of the formation.
Acidizing is an oilwell stimulation technique which permits an increase in the production output of a well and one feature which contributed towards the success of acidizing was the development of means of diversion.
Consequently, various techniques and diversion agents have from time to time been suggested with the aim of forcing the treatment fluid to reach the formation region that is to be treated. These have included diversion agents or chemical divergents, diversion with foam, mechanical diversion, and diversion with ball sealers.
The action of the diversion agents or chemical divergents is to form bridges over the formation pores, thereby causing a preferred reduction or elimination of the flow into those areas which have the highest receptivity and forcing the fluid into permeable regions, so that the treatment will become more uniform.
In gun-perforated wells, this bridging may be formed over the perforations, inside the perforations, or over the rock surface after the tunnels in the cement sheath. The resulting kind of plugging and the method of choice will depend on downhole conditions, type of formation, etc.
In diversion with chemical additives, the additives may be carried by the treatment fluid itself, or they may be conveyed in cushions, using a viscous fluid for transportation of the material. Usually these include sifted salt, solid organic acids, inert organic resins, oil-soluble waxes, and various combinations of these products.
This technique, however, has the following disadvantages: some good diverters tend to deteriorate beyond a certain high temperature: impurities contained in the product may become a source of damage to the formation; it may be difficult to choose the ideal divergent for a specific problem, and this may even include unavailability of the product.
As already mentioned, there are, other than this diversion technique, the techniques of diversion with foam, mechanical diversion and diversion with ball sealers.
Foam has long been known as a fluid capable of promoting diversion, but having the peculiarity of developing substantial loss of head while flowing through pipes and porous media. Depending on factors such as quality of the foam and pumping rate, high viscosities may develop as a result of the resistance to penetration into the porous medium. Accordingly, the introduction of a foam cushion between stages of an acidizing treatment ought to promote a diversion of the treatment.
This technique, however, has the drawback of requiring a source of gas supply and foam-making equipment; also, there is a high cost involved in the installation and handling of such equipment.
Mechanical diversion is the most positive method of diversion, since each stage of the treatment will act only on a secluded zone. Depending on the interval to be treated, it is not always possible to use mechanical diversion efficiently; this would be the case where the treatment is applied to intervals with a large area of perforations or intervals with large unperforated areas. In the former case, mechanical diversion would not be possible due to the widespread area involved, whereas in the latter case the moving of the equipment used for packing can cause substantial wear and consequent leaks, sometimes as serious as to require replacement. Besides, during trips the tool will leave the treated interval exposed to the completion fluid which may, in contact with the treatment fluids, jeopardize the acidizing achievements.
Regarding the conventional ball sealers, they are used in specific fracturing methods, since they are effective only at relatively high flow rates, which are normally not used in acidizing.
With the event of the availability of buoyant sealers, i.e. ball sealers which have a density lower than that of the treatment fluid, it became possible to widen the scope of their applicability due to the possibility of conveying them even at low flow rates.
However, such flow rates are not sufficient to carry the balls all the way to their target, i.e. into the perforations in the gunned intervals and, besides, this offers no selectivity in the treatment of distinct zones. Though certain flow rates may be capable of carrying the balls all the way downhole in the stream inside the tubing, they will upon reaching the casing cease to be sufficient to continue to convey them to their targets in the gunned intervals to be treated.
Regarding the state of the art developed in this field and related patents, we can mention, for instance, publications U.S. Pat. Nos. 4102401, 4139060, 4160482, 4195690, 4244425, 4279303, 4407368, 4421167, 4488599, and U.S. Pat. No. 4505334 4102401, 4244425, 4279303, 4407368 and U.S. Pat. No. 4505334 suggest various types of ball sealings as diversion agents; U.S. Part. No. 4160482 improves on the performance of buoyant ball sealers by controlling the speed of the treatment fluid; U.S. Pat. No. 4139060 and U.S. Pat. No. 4195690 suggest the use of two fluids of densities different from that of the ball sealers; and US 4421167 and US 4488599 are aimed at controlling the displacement of the balls by limiting their density according to the density of the treatment fluids.
Also regarding the prior art developed in this field, it was found that difficulties were met in controlling the correct setting of the balls in the interval to be treated; this could mean balls failing to reach their target or, in many cases, the occurrence of stimulation at intervals where stimulation would not be desired.
As a consequence, certain methods and devices have already been developed for the purpose of positioning the ball sealers selectively at adequate places so as to permit efficient diversion.
U.S. Pat. No. 4187909, for instance, attempts to solve this problem with an apparatus which carries the ball sealers inside it and which is run downhole to a position between two intervals to be treated. This apparatus has a means of permitting the flow of the balls only in the direction of the lower interval, thereby plugging it and leaving the upper interval free.
U.S. Pat. No. 4194561 describes an apparatus devised for proper conveyance of the ball sealers, to be positioned above the interval to be treated. It is provided with means which prevent the balls from flowing into upper intervals where plugging is not desired.
FIGS. 4 and 5 show a diversion operation using ball sealers according to the prior art, with FIG. 4 showing the use of heavy balls and FIG. 5 showing the operation with buoyant balls. In these Figures, as in all publications on patents and prior art, it can be observed that the treatment fluid together with the ball sealers develops certain flow rates in the stream down the production string (1) which slow down drastically and suddenly upon reaching the casing region (2). This is caused by the large increase in flow area and its consequence is a path which lacks efficiency in accomplishing the intended objective, i.e. in reaching the interval to be treated (5). The ball sealers, instead of flowing all the way to the intended interval (5), tend to locate themselves just above the bottom packer (4) if heavy, or in the region (6) just below the upper packer (7) if buoyant.